The invention is in the field of passive cooling devices capable of maintaining temperatures in the cryogenic range in sensing, optical and electronic devices, especially in space applications, in order to obtain optimum performance.
As the mechanical problems of delivery of equipment into space have been solved and such delivery has become more or less routine, space satellites and space vehicles have become increasingly more important as platforms for devices to accumulate and advance our knowledge of the earth, other planets, and astronomical phenomena. Such devices have become more and more complex and characteristically operate most efficiently at cryogenic temperatures. Maintenance of such temperatures has been difficult, especially in low earth orbit. Conventional cooling devices with mechanically active thermodynamic cycles are efficient but expensive, and unreliable over long periods of time. In addition they exact a heavy penalty in weight and reduce the usable payload of the launch vehicle.
Other cooling devices include solid cryogens, heat pipes and simple staged radiators. Solid cryogens, though effective, are difficult to contain and have limited operational life since they expend themselves by sublimation and evaporation into space. Heat pipes are useful as a heat transport means from remote locations but are only as effective as the means of dissipation of the transported heat at the heat rejection end of the heat pipe. Radiators have been used for this purpose but have normally been single-stage and subject to inefficiency as a result of the heat of the space vehicle transported by conductance through radiator insulation and supports structure. Staged radiators have appeared in the prior art but in all cases have either been in direct contact with the heat producing source or were primarily shields and secondarily radiators.
Ideally, a radiator should be totally, thermally isolated from the spacecraft structure. This is generally impractical, considering the constraints of the spacecraft launch envelope and the means to extend or provide the radiator with the proper orientation to avoid seeing the sun in certain phases of the orbit. Although insulation materials are constantly being improved, the minimum structure required for support, shielding and control will continue to introduce unwanted heating and thereby limit radiator efficiency.
Even where two separate radiators have been used to avoid the necessity for excessive shielding and to assure that one is always oriented away from the sun, the conductive heat load has been a significant factor in limiting the attainable low temperature. The weight of a second radiator and the associated means to support it, together with the complexity of providing controllable dual connections to the sensor, have been further penalties which make it an undesirable solution.